Minnesota - Remember the uproar in 1995 when school kids in Minnesota began finding frogs with extra limbs? The mutated amphibians looked like props in some sci-fi movie, and scientists quickly began searching for the culprit behind the deformities. Speculation centered on pesticides, increased UV radiation, and infection from parasites- which ultimately turned out to be the "villain."

But the question remained: why were these parasites—called trematodes—increasing in number and preying on frogs. According to a study published earlier this week in the Proceedings of the National Academy of Sciences, the leading cause of the problem is the runoff of phosphorous and nitrogen fertilizer, originating from agriculture (likely the monoculture corn and soybean farms of the Midwest), cattle grazing, and domestic runoff.

What's the connection? Through a process called eutrophication, the excess nutrients from animal manure and fertilizers cause more algae to grow in surface waters, like the pond where the kids first found the mutated frogs. The extra algae helps increase populations of snails (which feed on algae), as well as populations of the microscopic parasites (trematodes), which the snails eat and release into ponds. The trematodes form cysts on developing tadpoles, which can cause frogs to develop with missing, or in some cases multiple, limbs. The frog's predators then eat the frogs and the parasites, spreading the trematodes back into the ecosystem and relaunching the cycle.

But don't think you're safe from the effects of this pollution just because you're not an amphibian. "Since most human diseases involve multiple hosts, understanding how increased nutrient pollution affects freshwater and marine food webs to influence disease is an emerging frontier in ecological research," says Pieter Johnson, a water scientist at the University of Colorado at Boulder and the lead author of the study.